Crystallization Trends of Pyroxenes from Agpaitic Phonolites (Cantal, France)

R. Brousse and J. Ph. Rançon
Laboratoire de Pétrographic-Volcanologie, Université Paris XI, 91405 Orsay, France
Bureau de Recherches Géologiques et Minières, Départment Géothermie, BP 6009, 45060 Orléans Cédex, France

Abstract: Ca- and Na-rich pyroxene compositions of four agpaitic phonolites from the Cantal Volcano (France) have been studied. The pyroxenes from each sample show a range of compositions between X2+ pyroxencs (with X = Ca, Mg, Fe2+) and aegirine. The main substitution, X2+ by (Fe3+ + Na+), occurs when Na-Mg = 0.5, by which time Si is already equal to 2.0. The crystallization of Fe-Na-rich pyroxenes seems to be related to the increasing peralkalinity of the environment, in which the initially high temperature was suddenly reduced. Other factors, such as the structure of the liquid, are also considered.

Mineralogical Magazine; March 1984 v. 48; no. 346; p. 39-45; DOI: 10.1180/minmag.1984.048.346.06
© 1984, The Mineralogical Society
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